1. Field of the Invention
The invention relates to methods and systems for the treatment of exhaust gas generated by xe2x80x9clean burnxe2x80x9d operation of an internal combustion engine which are characterized by reduced tailpipe emissions of a selected exhaust gas constituent.
2. Background Art
Generally, the operation of a vehicle""s internal combustion engine produces engine exhaust that includes a variety of constituent gases, including carbon monoxide (CO), hydrocarbons (HC), and nitrogen oxides (NOx). The rates at which the engine generates these constituent gases are dependent upon a variety of factors, such as engine operating speed and load, engine temperature, spark timing, and EGR. Moreover, such engines often generate increased levels of one or more constituent gases, such as NOx, when the engine is operated in a lean-burn cycle, i.e., when engine operation includes engine operating conditions characterized by a ratio of intake air to injected fuel that is greater than the stoichiometric air-fuel ratio, for example, to achieve greater vehicle fuel economy.
In order to control these vehicle tailpipe emissions, the prior art teaches vehicle exhaust treatment systems that employ one or more three-way catalysts, also referred to as emission control devices, in an exhaust passage to store and release selected exhaust gas constituents, such as NOx, depending upon engine operating conditions. For example, U.S. Pat. No. 5,437,153 teaches an emission control device which stores exhaust gas NOx when the exhaust gas is lean, and releases previously-stored NOx when the exhaust gas is either stoichiometric or xe2x80x9crichxe2x80x9d of stoichiometric, i.e., when the ratio of intake air to injected fuel is at or below the stoichiometric air-fuel ratio. Such systems often employ open-loop control of device storage and release times (also respectively known as device xe2x80x9cfillxe2x80x9d and xe2x80x9cpurgexe2x80x9d times) so as to maximize the benefits of increased fuel efficiency obtained through lean engine operation without concomitantly increasing tailpipe emissions as the device becomes xe2x80x9cfilled.xe2x80x9d The timing of each purge event must be controlled so that the device does not otherwise exceed its capacity to store the selected exhaust gas constituent, because the selected constituent would then pass through the device and effect an increase in tailpipe emissions. The frequency of the purge is preferably controlled to avoid the purging of only partially filled devices, due to the fuel penalty associated with the purge event""s enriched air-fuel mixture.
The prior art has recognized that the storage capacity of a given emission control device is itself a function of many variables, including device temperature, device history, sulfation level, and the presence of any thermal damage to the device. Moreover, as the device approaches its maximum capacity, the prior art teaches that the incremental rate at which the device continues to store the selected constituent, also referred to as the instantaneous efficiency of the device, may begin to fall. Accordingly, U.S. Pat. No. 5,437,153 teaches use of a nominal NOx-storage capacity for its disclosed device which is significantly less than the actual NOx-storage capacity of the device, to thereby provide the device with a perfect instantaneous NOx-retaining efficiency, that is, so that the device is able to store all engine-generated NOx as long as the cumulative stored NOx remains below this nominal capacity. A purge event is scheduled to rejuvenate the device whenever accumulated estimates of engine-generated NOx reach the device""s nominal capacity.
The amount of the selected constituent gas that is actually stored in a given emission control device during vehicle operation depends on the concentration of the selected constituent gas in the engine feedgas, the exhaust flow rate, the ambient humidity, the device temperature, and other variables including the xe2x80x9cpoisoningxe2x80x9d of the device with certain other constituents of the exhaust gas. For example, when an internal combustion engine is operated using a fuel containing sulfur, the prior art teaches that sulfur may be stored in the device and may correlatively cause a decrease in both the device""s absolute capacity to store the selected exhaust gas constituent, and the device""s instantaneous constituent-storing efficiency. When such device sulfation exceeds a critical level, the stored SOx must be xe2x80x9cburned offxe2x80x9d or released during a desulfation event, during which device temperatures are raised above perhaps about 650xc2x0 C. in the presence of excess HC and CO. By way of example only, U.S. Pat. No. 5,746,049 teaches a device desulfation method which includes raising the device temperature to at least 650xc2x0 C. by introducing a source of secondary air into the exhaust upstream of the device when operating the engine with an enriched air-fuel mixture and relying on the resulting exothermic reaction to raise the device temperature to the desired level to purge the device of SOx.
Thus, it will be appreciated that both the device capacity to store the selected exhaust gas constituent, and the actual quantity of the selected constituent stored in the device, are complex functions of many variables that prior art accumulation-model-based systems do not take into account. The inventors herein have recognized a need for a method and system for controlling an internal combustion engine whose exhaust gas is received by an emission control device which can more accurately determine the amount of the selected exhaust gas constituent, such as NOx, stored in an emission control device during lean engine operation and which, in response, can more closely regulate device fill and purge times to optimize tailpipe emissions.
Under the invention, a method and system are provided for controlling an internal combustion engine that operates at a plurality of engine operating conditions characterized by combustion of air-fuel mixtures having different air-fuel ratios to generate engine exhaust gas, wherein the exhaust gas is directed through an exhaust treatment system including an emission control device that stores a selected exhaust gas constituent when the exhaust gas is lean and releases the stored selected exhaust gas constituent when the exhaust gas is rich, and a sensor operative to generate an output signal representative of a concentration of the selected constituent in the exhaust gas, such as NOx, exiting the device. The method includes determining a first value representative of an instantaneous concentration of the selected constituent in the engine exhaust gas during a lean operating condition; determining a second value representative of the instantaneous concentration of the selected constituent exiting the device based on the output signal generated by the sensor; and selecting an engine operating condition as a function of the first and second values. More specifically, in a preferred embodiment, the first value is estimated using a lookup table containing mapped values for the concentration of the selected constituent in the engine feedgas as a function of instantaneous engine speed and load. A lean operating condition is terminated, and a rich operating condition suitable for purging the device of stored selected constituent is scheduled, when the device efficiency, calculated based on the first and second values, falls below a predetermined minimum efficiency value. In this manner, the storage of the selected constituent in the device and, hence, the xe2x80x9cfill timexe2x80x9d during which the engine is operated in a lean operating condition, is optimized without reliance upon an accumulation model, in the manner characteristic of the prior art.
In accordance with another feature of the invention, the method preferably includes calculating a differential value based on the first and second values, with the differential value being representative of the amount of the selected constituent instantaneously stored in the device; and the differential value is accumulated over time to obtain a first accumulated measure representative of the total amount of the selected constituent which has been stored in the device during lean engine operation. The method further preferably includes calculating the amount of fuel, in excess of the stoichiometric amount, which is necessary to purge the device of both stored selected constituent and stored oxygen, based on the first accumulated measure and a previously stored value representing the amount of excess fuel necessary to purge only stored oxygen from the device. The method also preferably includes accumulating a value representative of an instantaneous amount of fuel supplied to the engine in excess of a stoichiometric amount during a purge event to obtain a second accumulated measure; and terminating the purge event when the second accumulated measure exceeds the total excess fuel value. In this manner, the invention optimizes the amount of excess fuel used to purge the device and, indirectly, the device purge time.
In accordance with another feature of the invention, the method preferably includes selecting a device-desulfating engine operating condition when the device""s calculated efficiency value falls below the minimum efficiency value and the first accumulated measure does not exceed a reference minimum-storage value for the selected constituent in the device. The method further preferably includes indicating device deterioration if a predetermined number of device-desulfating engine operating conditions are performed without any increase in a maximum value for the first accumulated measure.
In accordance with a further feature of the invention, the value representing the oxygen-only excess fuel amount is periodically updated using an adaption value which is itself generated by comparing the output signal of the sensor to a minimum-concentration reference value for the selected constituent upon terminating a scheduled purge. More specifically, the adaption value is generated as a function of any error between the output signal of the sensor and the minimum-concentration reference value.
The above object and other objects, features, and advantages of the present invention are readily apparent from the following detailed description of the best mode for carrying out the invention when taken in connection with the accompanying drawings.